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Osmosis

Osmosis occurs when water diffuses across a membrane that is freely permeable to water, but not to solutes. In this container, two solutions are separated by a membrane permeable only to water. The concentration of the large, yellow solute molecules is higher in solution B than it is in solution A since there are fewer solute molecules in solution A. The inverse is true of the water: the concentration of water molecules, represented by small blue spheres, is higher in A than in B. The way in which water will behave in this example is similar to how osmosis occurs in cells. The solute molecules are too large to pass through the pores of the membrane, however, water molecules freely move across the membrane.
Water molecules move down their concentration gradient, moving across the membrane from the side with a higher water concentration to the side with a lower water concentration. This increases the volume of B. Water movement continues until the solute concentration on both sides of the membrane is equal.